The present invention relates to disposable absorbent pads useful as disposable diapers, adult incontinence pads, sanitary napkins and the like. More particularly, the present invention relates to a nonwoven fabric top sheet for use in such absorbent pads.
Absorbent pads useful as disposable diapers customarily comprise a water-resistant backing sheet, a layer of absorbent or superabsorbent material, and a liquid pervious top sheet (often referred to as a coverstock, or, in diaper applications, a diaper liner) which is placed in contact with the body of the wearer.
To be useful as a top sheet in such a construction, the sheet material should provide fast initial passage of the liquid to be absorbed through the top sheet into the layer of absorbent material (i.e., exhibit a short "strike-through" time), while at the same time delaying or minimizing passage of the absorbed liquids from the layer of absorbent material back through the top sheet to the skin of the wearer (i.e., exhibit high "dryness" or low "surface rewet"). In addition, it is highly desirable that the top sheet material be flexible and soft to the touch. Furthermore, the top sheet material must have sufficient strength not to tear or rupture when wet.
Many different top sheet constructions have been disclosed for the manufacture of such absorbent articles, each typically being claimed to provide an improvement in one or more of the characteristics noted above. In the past, the top sheets typically comprised one or more nonwoven webs of natural or synthetic textile fibers such as rayon, polyamide, polyester, polypropylene, or the like, which were stabilized and secured together by a cured binder composition included within the webs. For example, U.S. Pat. No. 4,332,253 (Schoots) discloses such a top sheet wherein the nonwoven webs are stabilized and secured together with a polymeric binder composition containing at least 1% by weight of combined 2-ethyl hexyl acrylate. The inclusion of the 2-ethyl hexyl acrylate reportedly improves the dryness of the top sheet.
Similarly, U.S. Pat. No. 4,377,615 (Suzuki et al.) discloses a two-layer top sheet in which the nonwoven webs are stabilized and adhered together by an adhesive binder composition. The upper layer of the top sheet containing, as a principal element thereof, hydrophobic fibers and the lower layer containing a mixture of hydrophobic fibers and hydrophilic fibers, with the fibers in the lower layer being of coarser denier than the fibers in the upper layer, and the lower layer containing a smaller amount of the adhesive bonding material than the upper layer. This construction reportedly provides a top sheet exhibiting superior strike- through and dryness properties, an upper layer which is excellent in smoothness, touch and strength, and a lower layer which is excellent in bulkiness and cushion.
Currently, however, most top sheets are thin, low basis weight, carded or spunbond nonwoven fabrics formed by thermally bonding together synthetic thermoplastic fibers that have been made somewhat hydrophilic by the addition of wetting agents. In this regard, U.S. Pat. No. 4,668,566 (Braun) discloses a two-layer top sheet comprising a first layer of polypropylene fibers thermally bonded to a second layer of polyethylene fibers, at discrete locations, which is formed by passing the layers between heated rolls, one having a smooth surface and one having a raised pattern thereon. Reportedly, a top sheet made in this manner exhibits increased softness and tensile strength. However, in order to obtain good strike-through, the polyethylene filaments must be coated with a wetting agent to increase their hydrophilicity.
Similarly, U.S. Pat. No. 4,704,112 (Suzuki et al.) discloses a two-layer top sheet comprising a first layer, having a pattern of apertures, composed of hydrophobic fibers in an amount of 70 to 100% by weight and hydrophilic fibers in an amount of 0 to 30% by weight, thermally bonded to a second layer, having no apertures, composed of hydrophilic fibers in an amount of 50 to 100% by weight and hydrophobic fibers in an amount of 0 to 50% by weight. Useful hydrophobic fibers are said to include polyester, polypropylene, polyethylene, acryl and polyurethane fibers, and useful hydrophilic fibers are said to include rayon fibers, cotton fibers and synthetic fibers such a polyester in which the fiber surface has been imparted with a hydrophilic nature. Reportedly, such a top sheet construction provides an improvement in the balance achieved between surface rewet and strike-through characteristics.
Although the top sheet constructions referred to above indicate that significant advances have been made in the formulation of top sheets exhibiting the desired characteristics, there remain significant disadvantages associated with each of these constructions. For example, the adhesive binder compositions included within the adhesively bonded constructions increase the cost of the top sheet. Additionally, since these binder resin compositions are often applied in the form of aqueous dispersions, the energy required for drying the nonwoven webs and curing the binder resin further increases the cost of manufacture. Moreover, while the thermally bonded constructions avoid the disadvantages associated with the use of adhesive binder resins, they undesirably require the use of hydrophobic thermoplastic fibers which introduce at least three disadvantages. First, the hydrophobic thermoplastic fibers must be treated with wetting agents to provide sufficient hydrophilicity to achieve adequate strike-through. Second synthetic thermoplastic fibers are often judged to provide less comfort than natural hydrophilic fibers. Third, the hydrophobic thermoplastic fibers are not biodegradable, and, thus, contribute to the growing environmental problems of waste disposal and management.
It is, therefore, an object of the present invention to provide a thermally bonded nonwoven top sheet that provides a superior balance of strike-through and dryness properties, and which comprises a significant proportion of natural, biodegradable, hydrophilic fibers.